Surface emitting dfb laser structures for broadband communication systems and array of same

ABSTRACT

A surface emitting semiconductor laser ( 10 ) is shown having a semiconductor lasing structure having an active layer ( 22 ), opposed cladding layers contiguous to said active layer, a substrate ( 17 ), and electrodes ( 12,14 ) by which current can be injected into the semiconductor lasing structure. Also included is a second or higher order distributed diffraction grating ( 24 ) having periodically alternating elements, each of the elements being characterized as being either a high gain element ( 26 ) or a low gain element ( 28 ). Each of the elements has a length, the length of the high gain element and the length of the low gain element together defining a grating period, where the grating period is in the range required to produce an optical signal in the optical telecommunications signal band. The total length of the high gain elements is no more than the total the lengths of the low gain elements. A single laser structure may be provided or an array of side by side laser structures on a common substrate is also provided. In a further aspect a method of testing laser structures on wafer is provided.

FIELD OF THE INVENTION

This invention relates generally to the field of telecommunications andin particular to optical signal based telecommunication systems. Mostparticularly, this invention relates to lasers, such as semiconductordiode lasers, for generating carrier signals for such opticaltelecommunication systems.

BACKGROUND OF THE INVENTION

Optical telecommunications systems are rapidly evolving and improving.In such systems individual optical carrier signals are generated, andthen modulated to carry information. The individual signals are thenmultiplexed together to form dense wavelength division multiplexed(DWDM) signals. Improvements in optical technology have led to closerspacing of individual signal channels, such that it is now common for 40signal channels to be simultaneously deployed in the C-band, with 80 oreven 160 simultaneous signal channels in the combined C+L bandsbeginning to be deployed in the near future.

Each signal channel requires an optical signal carrier source and intelecommunications the signal carrier source is typically a laser. Asthe number of DWDM signal channels increases, the number of signalcarrier sources needed also increases. Further, as optical networks pushoutward from the data-dense long haul backbones to the data-light edgeor end user connections, a vast number of new network nodes are needed,potentially each with the multiple signal carrier sources required forDWDM. As well, the cost of supplying signal carrier sources becomes anissue as a function of data traffic since the data density is less, thecloser to edge of the network one is. A number of different lasersources are currently available. These include various forms of fixed,switchable or tunable wavelength lasers, such as Fabry-Perot,Distributed Bragg Reflector (DBR), Vertical Cavity Surface EmittingLasers (VCSEL) and Distributed Feedback (DFB) designs. Currently themost common form of signal carrier source used in telecommunicationapplications are edge emitting index coupled DFB laser sources, whichhave good performance in terms of modulation speed, output power,stability, noise and side mode suppression ratio (SMSR). In addition, byselecting an appropriate semiconductor material and laser design,communication wavelengths can be readily produced. In this sense SMSRrefers to the property of DFB lasers to have two low thresholdlongitudinal modes having different wavelengths at which lasing canoccur, of which one is typically desired and the other is not. SMSRcomprises a measure of the degree to which the undesired mode issuppressed, thus causing more power to be diverted into the preferredmode, while also having the effect of reducing cross-talk from theundesired mode emitting power at the wavelength of another DWDM channel.A drawback of edge emitting DFB laser signal sources is that the beamshape is in the form of a short stripe, strongly diverging in twodimensions with differing divergence angles due to the small aperture ofthe emitting area, which requires a spot converter to couple the signalto a single mode fibre. The necessary techniques are difficult and canbe lossy, resulting in increased cost.

Although they can achieve good performance once finished and coupled tothe fibre, edge emitting DFB lasers have several fundamentalcharacteristics that make them inefficient to produce and hence moreexpensive. More specifically, large numbers of edge emitting DFB lasersare currently produced simultaneously on a single wafer. However, theyield of viable edge emitting DFB lasers (i.e. those which meet thedesired signal output specifications) obtained from a given wafer can below due to a number of factors in the final fabrication or packagingsteps. Specifically, once formed, the individual DFB laser must becleaved off the wafer. The cleaving step is then followed by anend-finishing step, most usually the application of an anti-reflectivecoating to one end and a high-reflective coating to the other. Ifsymmetric coatings (usually anti-reflective) are applied to bothsurfaces, then the two main modes of the laser are degenerate and thereno a priori discrimination between modes, leading to poor control of theSMSR and therefore poor single mode yield. The asymmetry introduced bydifferent end coatings helps to give preference to one mode over theother, thus improving the SMSR. However, even though single modeoperation is improved, the wavelength of the DFB laser is still afunction of the phase of the grating where it was cleaved at the end ofthe laser cavity. Uncertainty in the phase introduced by the cleavingstep results in poor control of the lasing wavelength. Therefore lasersproduced in this way generally have poor single mode yield, wavelengthyield, or both and are not optimal for use in DWDM systems.

An important aspect of the fabrication of edge emitting DFB lasers isthat the laser can only be tested by injecting a current into the lasingcavity after the laser has been completely finished, including cleavingfrom the wafer and end-coating. This compounds the inefficiency of suchlow yields from the wafer due to multimode behaviour (poor SMSR) orincorrect wavelength.

Designs intended to increase the yield of single mode edge emitting DFBlasers have been proposed, most notably by introducing a quarterwavelength phase shift in the centre of the laser cavity combined withanti reflection coating both facets of the cavity. This structuresuffers from spatial hole burning as a result of the intense fieldgenerated in the region of the phase shift. This limits the output powerof the device. Further, the laser is very sensitive to even smallreflections from the facets, adding a source of instability anddifficulty due to the need for high quality anti-reflection coatings onthe facets.

Other methods for lifting the degeneracy of the modes in DFB lasersinvolve introducing an imaginary, or complex, term to the couplingcoefficient. One way this has been achieved is to fabricate the gratingwithin either the active gain layer (a so-called gain-coupled design) orwithin an absorbing layer that is within the optical mode field (aloss-coupled design). These designs have only recently been practicaldue to advances in the required semiconductor fabrication techniques.Both gain and loss coupled DFB lasers exhibit a significantly reducedsensitivity to the random phase induced by the cleaving step as well asother benefits including high single mode yield, narrower linewidth, andimproved ac response (i.e. they can be modulated at higher frequencies).Gain and loss coupled designs still, however, require cleaving andcoating of the facets before the chip can be tested. As well, theemission is still from the edge and coupling into a fibre remains aproblem.

Both surface emission and single mode operation through complex couplinghave been achieved by using a second or higher order grating instead ofthe more common first order grating. In the case of a second ordergrating, the resulting radiation loss from the surface of the laser isdifferent for the two modes, thus lifting the degeneracy and resultingin single mode operation, as described by R. Kazarinov and C. H. Henryin IEEE, J. Quantum Electron., vol. QE-21, pp. 144-150, February 1985.With an index coupled second order grating, the spatial profile of thelasing mode is dual-lobed with a minimum at the centre of the lasercavity. The suppressed mode in this instance is a single-lobedGaussian-like profile peaked at the centre of the cavity. Note that theprofile is Gaussian-like in both directions but is asymmetric in thatthe Gaussian width is in general much larger along the axis of the laseras compared to the Gaussian width transverse to the laser. This lattermode, while being beneficial to most applications, is perhaps even morecritical in the field of telecommunications because it more closelymatches the mode diameter and numerical aperture of a single modeoptical fibre and can therefore be efficiently coupled into the fibre.The dual-lobed shape can only be coupled to a fibre with poorefficiency.

Attempts have been made in the art to alter the laser such that thesingle-lobed mode of surface emitting DFB lasers becomes the dominantmode, but without much success. For example, U.S. Pat. No. 5,970,081teaches a surface emitting, index coupled, second order grating DFBlaser structure that introduces a phase shift into the laser cavity bymeans of constricting the shape of the wave guide cavity structure inthe middle such that the lasing mode is the preferred approximatelyGaussian mode. This method is difficult to implement due to thelithography involved and the design leads to a deterioration of otherspecifications related to an increase in spatial hole burning in theregion of the phase shift. Furthermore, the lower efficiency of theradiation coupling and low coupling coefficient of the index-coupledversus the gain coupled design lead to a low power from the surface aswell as relatively high threshold current for the device.

Similarly, U.S. Pat. No. 4,958,357 directly introduces a phase shift ina surface emitting, index coupled, second order grating DFB laser withsimilar difficulties resulting. While purporting to offerwafer-evaluation and an elimination of facet-cleaving due to surfaceemission, this patent teaches a complex structure which is difficult tobuild and even more difficult to control. Due to a cusp in the opticalintensity at the location of the phase shift spatial hole burningresults. While various schemes are proposed to mitigate spatial holeburning these add complexity and in any event are not successful. Thus,scale-up is limited by spatial hole burning.

Outside of the telecommunications field, an example of a surfaceemitting DFB laser structure is found in U.S. Pat. No. 5,727,013. Thispatent teaches a single lobed surface emitting DFB laser for producingblue/green light where the second order grating is written in anabsorbing layer within the structure or directly in the gain layer.While interesting, this patent does not disclose how the grating affectsfibre coupling efficiency (since it is not concerned with any telecomapplications). This patent also fails to teach what parameters controlthe balance between total output power and fibre coupling efficiency orhow to effectively control the mode. Lastly, this patent fails to teacha surface emitting laser which is suitable for telecommunicationwavelength ranges.

More recently, attempts have been made to introduce vertical cavitysurface emitting lasers (VCSELs) with performance suitable for thetelecommunications field. Such attempts have been unsuccessful for anumber of reasons. Such devices tend to suffer from a difficulty infabrication due to the many layered structure required as well as a lowpower output due to the very short length of gain medium in the cavity.The short cavity is also a source of higher noise and broader linewidth.The broader linewidth limits the transmission distance of the signalfrom these sources due to dispersion effects in the fibre.

SUMMARY OF THE INVENTION

What is needed is a surface emitting laser structure which is bothsuitable for telecommunications applications and which avoids thedefects of the prior art. More particularly what is needed is a laserstructure where the mode is controlled precisely and efficiently topermit fibre coupling and yet which can be made using conventionallithographic techniques in the semiconductor art. An object of thepresent invention is to provide a low-cost optical signal source that iscapable of generating signals suitable for use in the optical broadbandtelecommunications signal range. Most preferably such a signal sourcewould be in the form of a semiconductor laser which can be fabricatedusing conventional semiconductor manufacturing techniques and yet whichwould have higher yields than current techniques and thus can beproduced at a lower cost. It is a further object of the presentinvention that such a signal source would have enough power, wavelengthstability and precision for broadband communications applications. Whatis also desired is a semiconductor laser signal source having a signaloutput which is easily and efficiently coupled to an optical fibre. Sucha device would also preferably be fabricated as an array on a singlewafer-based structure and may be integrally and simultaneously formed orfabricated with adjacent structures such as signal absorbing adjoiningregions and photodetector devices.

A further feature of the present invention relates to efficiencies inmanufacturing. The larger the number of arrayed signal sources thegreater the need for a low fault rate fabrication. Thus, for example, aforty source array fabricated at a yield of 98% per source will producean array fabrication yield of only 45%. Thus, improved fabricationyields are important to cost efficient array fabrication.

A further aspect of the invention is that each laser source of the arraycan be set to the same or, more usefully, to different wavelengths andmost preferably to wavelengths within the telecommunications signalbands. Most preferably such a device would also provide a simple andeffective means to confine the output signal to also help the fibrecoupling efficiencies. Further such a device could have a built indetector that, in conjunction with an external feedback circuit, couldbe used for fine wavelength tuning and signal maintenance.

Therefore according to a first aspect of the present invention there isprovided a surface emitting semiconductor laser comprising:

a semiconductor lasing structure having an active layer, opposedcladding layers contiguous to said active layer, a substrate, arefractive index structure to laterally confine an optical mode volumeand electrodes by which current can be injected into said semiconductorlasing structure, and

a second order distributed diffraction grating having periodicallyalternating grating elements, each of said grating elements beingcharacterized as being either a high gain element or a low gain element,where the low gain element may exhibit low gain as compared to the highgain element, no gain or absorption, each of said grating elementshaving a length, the length of the high gain element and the length ofthe low gain element together defining a grating period, said gratingperiod being in the range required to produce an optical signal in thewavelength band of optical telecommunications signals, wherein thelength of the high gain grating element is no more than 0.5 times thelength of the grating period.

According to a second aspect of the present invention there is alsoprovided a method of fabricating semiconductor lasers, said methodcomprising the steps of:

forming a plurality of semiconductor laser structures by forming, insuccessive layers on a substrate;

a first cladding layer, an active layer and a second cladding layer on awafer;

forming a plurality of second order distributed diffraction gratings onsaid wafer;

forming electrodes on said wafer for injecting current into each of saidgratings; and

testing said semiconductor structures by injecting current into saidstructures in said wafer form.

According to a third aspect of the present invention there is alsoprovided a surface emitting semiconductor laser for producing outputsignals of defined spatial characteristics said laser comprising;

a semiconductor lasing structure having an active layer, opposedcladding layers contiguous to said active layer, a substrate andelectrodes by which current can be injected into said semiconductorlasing structure to produce an output signal in a telecommunicationsband and a second order distributed diffraction grating sized and shapedto provide, upon the injection of current into the lasing structure, alower gain threshold to a single lobed mode than the gain thresholdprovided to any other mode wherein said single lobe mode lases tofacilitate coupling said output signal to an optical fibre.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made, by way of example only,.to preferredembodiments of the present invention by reference to the attachedfigures, in which:

FIG. 1 is a side view of one embodiment of a surface emittingsemiconductor laser according to the present invention having a secondorder grating formed in a gain medium;

FIG. 2 is an end view of the embodiment of FIG. 1;

FIG. 3 is a schematic plot of the gain coupling coefficient K_(g),radiation coupling coefficient K_(r), index coupling coefficient K_(i),the imaginary part of the total coupling coefficient K_(g)+K_(r), andthe coupling strength (K_(g)+K_(r))/K_(i) vs. the duty cycle of a highgain element as compared to the grating period;

FIG. 4 is a side view of a second embodiment of a surface emittingsemiconductor laser according to the present invention having a secondorder grating formed in an absorbing or loss layer;

FIG. 5 is an end view of the embodiment of FIG. 4;

FIG. 6 is a schematic plot of mode 1 and mode 2 profiles of opticalnear-field intensity vs. distance along the laser cavity;

FIG. 7 is a top view of a further embodiment of the present inventionshowing termination regions in the form of absorbing regions at eitherend of a laser cavity;

FIG. 8 is top view of a further embodiment of the invention of FIG. 7wherein one of said termination regions is a detector;

FIG. 9 is a top view of a further embodiment of the present inventionwherein the termination regions include first order grating sections;and

FIG. 10 is top view of an array of surface emitting semiconductor laserstructures on a common substrate for generating wavelengths 1 to N.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a side view of one embodiment of a surface emittingsemiconductor laser structure 10 according to the present invention,while FIG. 2 is an end view of the same structure. The laser structure10 is comprised of a number of layers built up one upon the other using,for example, standard semiconductor fabrication techniques. It will beappreciated that the use of such known semiconductor fabricationtechniques for the present invention means that the present inventionmay be fabricated efficiently in large numbers without any newmanufacturing techniques being required.

In this disclosure the following terms shall have the followingmeanings. A p-region of a semiconductor is a region doped with electronacceptors in which holes (vacancies in the valence band) are thedominant current carriers. An n-region is a region of a semiconductordoped so that it has an excess of electrons as current carriers. Anoutput signal means any optical signal which is produced by thesemiconductor laser of the present invention. The mode volume means thevolume in which the optical mode exists, namely, where there is light(signal) intensity. For the purposes of this disclosure, a distributeddiffraction grating is one in which the grating is associated with theactive gain length or absorbing length of the lasing cavity so thatfeedback from the grating causes interference effects that allowoscillation or lasing only at certain wavelengths, which theinterference reinforces.

The diffraction grating of the present invention is comprised of gratingor grid elements, which create alternating gain effects. Two adjacentgrating elements define a grating period. The alternating gain effectsare such that a difference in gain arises in respect of the adjacentgrating elements with one being a relatively high gain effect and thenext one being a relatively low gain effect. The present inventioncomprehends that the relatively low gain effect may be a small butpositive gain value, may be no actual gain or may be an absorbing ornegative value. Thus, the present invention comprehends any absolutevalues of gain effect in respect of the grating elements, provided therelative difference in gain effect is enough between the adjacentgrating elements to set up the interference effects of lasing at onlycertain wavelengths. The present invention comprehends any form ofgrating that can establish the alternating gain effects described above,including loss coupled and gain coupled gratings and carrier blockinggratings whether in the active region or not.

The overall effect of a diffraction grating according to the presentinvention may be defined as being to limit laser oscillation to eitherone or both of two longitudinal lasing modes, with various additionaltechniques being employed to further design the laser such that only asingle longitudinal mode is stable, giving the laser a narrow line widthwhich may be referred to as a single-mode output signal.

As shown in FIG. 1, the two outside layers 12 and 14 of the laserstructure 10 are electrodes. The purpose of the electrodes is to be ableto inject current into the laser structure 10. It will be noted thatelectrode 12 includes an opening 16. The opening 16 permits the opticaloutput signal to pass outward from the laser structure 10, as describedin more detail below. According to the present invention, the openingcan also be formed on the opposite electrode 14. As well, although aridge waveguide device is shown, the present invention comprehends otherwaveguide structures such as, for example, a buried heterostructure.Although an opening is shown, the present invention comprehends the useof a continuous electrode, providing the same is made transparent, atleast in part, so as to permit the signal generated to pass out of thelaser structure 10. Simple metal electrodes, having an opening 16, havebeen found to provide reasonable results and are preferred due to easeof fabrication and low cost.

Adjacent to the electrode 12 is an n+InP substrate, or wafer 17.Adjacent to the substrate 17 is a buffer layer 18 which is preferablycomprised of n−InP. The next layer is a confinement layer 20 formed fromn−InGaAsP. The generic composition of this and other quaternary layersis of the form In_(x)Ga_(1-x)As_(y)P_(1-y) while ternary layers have thegeneric composition In_(1-x)Ga_(x)As. The next layer is an active layer22 made up of alternating thin layers of active quantum wells andbarriers, both comprised of InGaAsP or InGaAs. As will be appreciated bythose skilled in the art InGaAsP or InGaAs is a preferred semiconductorbecause these semiconductors, within certain ranges of composition, arecapable of exhibiting optical gain at wavelengths in the range of 1200nm to 1700 nm or higher, which comprehends the broadband optical spectraof the 1300 nm band (1270-1330 nm), the S-band (1468-1525 nm), theC-band (1525 nm to 1565 nm) and the L-band (1568 to 1610 nm). Othersemiconductor materials, for example GaInNAs, InGaAlAs are alsocomprehended by the present invention, provided the output signalgenerated falls within the broadband range. Other relevant wavelengthranges of telecommunications importance for which devices following thisinvention could be designed using appropriate material compositions (forexample InGaAs/GaAs) are the region from 910 to 990 nm (whichcorresponds to the most commonly encountered wavelength range forpumping optical amplifiers and fibre lasers based on Er, Yb or Yb/Erdoped materials) and near 850 nm (commonly used for short range datatransmission). In the embodiment of FIG. 1, a diffraction grating 24 isformed in the active layer 22. The grating 24 is comprised ofalternating high gain portions 26 and low gain portions 28. Mostpreferably, the grating 24 is a regular grating, namely has a consistentperiod across the grating, and is sized, shaped and positioned in thelaser 10 to comprise a distributed diffraction grating as explainedabove. In this case, the period of the grating 24 is defined by the sumof a length 30 of one high gain portion 26 and a length 32 of theadjacent low gain portion 28. The low gain portion 28 exhibits low or nogain as compared to the high gain portion as in this region most or allof the active structure has been removed. According to the presentinvention, the grating 24 is a second order grating, namely, a gratingwith a period equal to the wavelength of the desired wavelength in thesemiconductor medium, which results in output signals in the form ofsurface emission. Higher order gratings also display surface emission,but with more beams at different angles from higher orders, thusdecreaseing efficiency into the desired output beam. As can now beappreciated, since the grating 24 of this embodiment is formed in theactive gain layer it is referred to as a gain coupled design.

The next layer above the grating 24 is a p−InGaAsP confinement layer 34.Located above the confinement layer 34 is a p−InP buffer region 36.Located above layer 36 is a p−InGaAsP etch stop layer 38. Then, a p−InPcladding layer 40 is provided surmounted by a p⁺⁺-InGaAs cap layer 42.

It will be understood by those skilled in the art that a semiconductorlaser built with the layers configured as described above can be tunedto produce an output signal of a predetermined wavelength as thedistributed feedback from the diffraction grating written in the activelayer renders the laser a single mode laser. The precise wavelength ofthe output signal will be a function of a number of variables, which arein turn interrelated and related to other variables of the laserstructure in a complex way. For example, some of the variables affectingthe output signal wavelength include the period of the grating, theindex of refraction of the active, confinement, and cladding layers(which in turn typically change with temperature as well as injectioncurrent), the composition of the active regions (which affects the layerstrain, gain wavelength, and index), and the thickness of the variouslayers that are described above. Another important variable is theamount of current injected into the structure through the electrodes.Thus, according to the present invention by manipulating these variablesa laser structure can be built which has an output with a predeterminedand highly specific output wavelength. Such a laser is useful in thecommunications industry where signal sources for the individual channelsor signal components which make up the DWDM spectrum are desired. Thusthe present invention comprehends various combinations of layerthickness, gain period, injection current and the like, which incombination yield an output signal having a power, wavelength andbandwidth suitable for telecommunications applications.

However, merely obtaining the desired wavelength and bandwidth is notenough. A more difficult problem solved by the present invention is toproduce the specific wavelength desired from a second order grating (andthus, as a surface emission) in such a manner that it can be controlledfor efficient coupling, for example, to an optical fibre. The spatialcharacteristics of the output signal have a big effect on the couplingefficiency, with the ideal shape being a single mode, single-lobedGaussian. For surface emitting semiconductor lasers the two primarymodes include a divergent dual-lobed mode, and a single-lobed mode. Theformer is very difficult to couple to a single mode fibre as isnecessary for most telecommunications applications because the fibre hasa single Gaussian mode. Conversely, the single lobed mode of the laseris considerably easier and more efficient to couple to a fibre, sincethe peak of the energy intensity is located centrally and it much moreclosely has the shape of the fibre mode. According to the presentinvention a surface emitting laser structure can be built in which thepreferred mode reliably dominates.

As noted above, SMSR refers to the suppression of the unwanted mode infavour of the wanted mode(s). According to the present invention, toachieve good SMSR operation from the surface of the laser 10 requirescareful attention to the design of the duty cycle of the grating 24 andthus to the spatial modulation of the gain through the active layer 22.In this description, the term duty cycle means the fraction of thelength of one grating period that exhibits high gain as compared to thegrating period. In more simple terms, the duty cycle may be defined asthe portion of the period of the grating 24 that exhibits high gain.This parameter of duty cycle is controlled in gain coupled lasers, suchas illustrated in FIG. 1, by etching away portions of the active layers,with the remaining active layer portion being the duty cycle.Alternatively, the active gain layers can be left intact and the gratingcan be etched into a current blocking layer, with the fraction ofcurrent blocking layer etched away corresponding to the duty cycle.

In FIG. 1, it can now be understood that the second order distributeddiffraction grating is written by etching the gain medium to form thegrating 24. As a result, the two fundamental modes of the semiconductorlaser 10 exhibit different surface radiation losses (which is the outputof the laser) and therefore have very different gains. Only one mode(the mode with the lowest gain threshold) will lase, resulting in goodSMSR. The present invention comprehends that the desired lasing mode isthe single lobed mode that has a profile which is generally Gaussian inappearance. In this way the lasing mode can be easily coupled to afibre, since the profile of the power or signal intensity facilitatescoupling the output signal to a fibre.

To have the desired single-lobed mode as the single lasing modeaccording to the present invention, it is important to limit the dutycycle to a specific range of values. The reason for this is explainedwith reference to FIG. 3, which shows the dependence of the gain,radiation and index coupling coefficients (K_(g), K_(r), and K_(i)respectively), the imaginary part of the total coupling coefficient(K_(g)+K_(r)) and the coupling strength ((K_(g)+K_(r))/K_(i)), as afunction of the duty cycle of the high gain portion of a distributedsecond order diffraction grating. Note the total coupling coefficient isdefined as K_(i)+j(K_(g)+K_(r)), where here j is (−1)^(½). The importantfeatures to note are that the index and gain coupling coefficients aresinusoidal while the radiation coupling coefficient is Gaussian-like andnegative. The total coupling coefficient, taken with the cavity lossesK_(t)=K+i(K_(g)+K_(r)) has as the imaginary part K_(g)+K_(r) while thecoupling strength (K_(g)+K_(r))/K_(i) is a measure of the imaginary tothe real part of the total coupling coefficient. The real part of thetotal coupling coefficient (K_(i)), taken with the effective cavitylosses, largely determines the gain threshold while the couplingstrength is a good indication of the degree of discrimination betweenthe two fundamental modes since the imaginary part of the total couplingcoefficient favours one mode over the other while the real part (K_(i))does not discriminate between the two.

Of the two fundamental modes of the laser, the one that will lase willbe the one with the lowest gain threshold. Referring to the curves inFIG. 3 for the case of a second order gain coupled laser design asdescribed above, when K_(g)+K_(r) is positive the single-lobed mode willhave the lowest gain threshold while the dual-lobed mode will have alower threshold when the value is negative. Since Kr is negative, thesum K_(g)+K_(r) will always be negative for values of duty cycle above0.5. The cross-over point will always be less than 0.5, only approaching0.5 when K_(g)>>K_(r). Therefore the upper limit to duty cycle toachieve desired operation is 0.5. The mode discrimination is enhancedfor larger values of K_(g)+K_(r), showing that optimal values of dutycycle are near 0.25. It can be seen that the coupling strength over thisregion of duty cycles is relatively flat and therefore is not a majorfactor provided the value is sufficiently large. Another issue that mustbe considered in a final design is that with the lowering of the dutycycle there is less gain material present and so higher material gainsare required as the duty cycle is lowered. This situation pushes optimalduty cycles to be as large as possible to alleviate the requirements onmaterial gain. Taken all together, this invention comprehends a usefulregion of duty cycle to be between about 15% and 35%.

In addition to the mode discrimination (SMSR) due to design of the lasercavity, we also consider the contribution to SMSR due to the fibrecoupling step. Since only the generally Gaussian mode is easily coupledto a fibre, a significant improvement in SMSR can be realized with thepower of the other mode not being coupled to the fibre. Taken togetherwith the high discrimination between modes due to the cavity design, theoverall SMSR of the laser is excellent.

Turning to FIG. 2, a side-view of the laser structure of FIG. 1 isshown. As can be seen in FIG. 2, the electrodes 12 and 14 permit theapplication of a voltage across the semiconductor laser structure 10 toencourage lasing as described above. Further, it can be seen that theridge formed by the top layers serves to confine the optical modelaterally to within the region through which current is being injected.While a ridge waveguide is shown in this embodiment it is comprehendedthat a similar structure could be fabricated using a buriedheterostructure sized and shaped to confine the carriers and opticalfield laterally.

Other forms of gain coupled designs are comprehended as a means toimplement the present invention. For example instead of etching theactive region as described above, a further highly n-doped layer can bedeposited above the active layer and a grating can be made in thislayer. This layer would then be not active optically and thus neitherabsorbs nor exhibits gain. Instead, it blocks charge carriers from beinginjected into the active layer wherever it has not been etched away.This structure for an edge emitting gain coupled laser is taught in C.Kazmierski, R. Robein, D. Mathoorasing, A. Ougazzaden, and M. Filoche,IEEE, J. Select Topics Quantum Electron., vol. 1, pp. 371-374, June1995. The present invention, comprehends modifying such a structure tolimit the carrier blocking layer to having openings in it with a dutycycle of less than 0.5 preferably in the range of 0.15 to 0.35 and mostpreferably about 0.25 (i.e. about 0.75 blocking).

Turning to FIG. 4, a further embodiment of a surface emittingsemiconductor laser structure 100 is shown. In this embodiment,electrodes 112 and 114 are provided at the top and bottom. Adjacent tothe electrode 112 is an n+InP substrate 116 followed by a n—InP buffer118. An opening 117 is provided in electrode 112. Again, the openingcould also be in the opposite electrode 114. A first confinementn−InGaAsP layer 120 is provided above which is located an active region122 comprised of InGaAsP or InGaAs quantum well layers separated byInGaAsP or InGaAs barrier layers. Then, a p−InGaAsP confinement region124 is provided with a p−InP buffer region 126 there-above. A grating125 is formed in the next layer, which is a p− or n−InGaAs or InGaAsPabsorption layer 128. A further p−InP buffer layer 130 is followed by ap−InGaAsP etch stop layer 132. Then, a p−InP cladding layer 134 isprovided along with a p⁺⁺-InGaAs cap layer 136 below the electrode 114.As will now be appreciated, this embodiment represents a second (orhigher) order grating which is formed by providing an absorbing layerand etching or otherwise removing the same to form a loss coupleddevice. The grating 125 is comprised of a periodically reoccurring lossor absorption elements. When taken together with the continuous gainlayer 122 (even though the gain layer is not on the same level as theabsorption layer) this grating 125 can be viewed as a grating havingperiodically repeating high gain elements 138 and low gain (which may beno gain or even net loss) elements 140. The combination of any one highgain element 138 and one low gain element 140 defines a period 142 forsaid grating 125.

FIG. 5 shows the semiconductor laser structure of FIG. 4 in end view. Ascan be noted, a current can be injected through the electrodes 112 and114 to the semiconductor laser structure 100 for the purpose of causinglasing in as described above. As in FIG. 2, the ridge provides thelateral confinement -for the optical field. FIG. 6 is a schematic of anoptical near-field intensity versus the distance along the laser cavity,and is generally applicable to both of the previously describedembodiments. As shown, at the middle of the laser cavity, the mode 1(the wanted generally Gaussian shaped) field intensity is at a peak 144,whereas the mode 2 (the unwanted divergent dual lobed) field intensityis at a minimum 146. Thus, at the middle of the laser cavity the opticalfield is much more intense in the mode 1 or Gaussian profile. This FIG.6 therefore illustrates the highly effective side mode suppressionarising from the controlled duty cycle of the present invention. Furtherit illustrates the need for the opening 16 in the electrode 12 in themiddle of the cavity to let out the signal as shown in FIG. 1. As notedearlier, this opening can be located on either electrode.

FIG. 7 shows a top view of a further embodiment of the presentinvention, where the grating region 150 includes finished end portions152, 154 for improved performance. As can be seen the grating 150 can bewritten onto a wafer 156 (shown by break line 158) using knowntechniques. The grating 150 so written can be surrounded by an adjoiningregion 160 which separates and protects the grating 150. Because thepresent invention is a surface emitting device, rather than cleaving thegrating end portions as in the prior art edge emitting lasers, thepresent invention contemplates cleaving, to the extent necessary, in thenon-active adjoining region 160. Thus, no cutting of the grating 150occurs during cleaving and the properties of each of the gratings 150can be specifically designed, predetermined and written according tosemiconductor lithographic practices. Thus, each grating can be madewith an integral number of grating periods and each adjacent grating onwafer 156 can be written to be identical or different from itsneighbours. The only limit of the grating is the writing ability of thesemiconductor fabrication techniques. Importantly, unlike the prior artedge emitting semiconductor lasers the grating properties will notchange as the laser structures are packaged.

The present invention further comprehends making the grating terminationportions 152, 154 absorbing regions. This is easily accomplished by notinjecting current into the termination regions as the active layer isabsorbing when not pumped by charge injection. As such, these regionswill strongly absorb optical energy produced and emitting in thehorizontal direction, thus fulfilling the function of theanti-reflective coatings of the prior art without further edge finishingbeing required. Such absorbing regions can be easily formed as thelayers are built up on the wafer during semiconductor manufacturingwithout requiring any additional steps or materials. In this manner afinishing step required in the prior art is eliminated, making laserstructures 10 according to the present invention more cost efficient toproduce than the prior art edge emitting lasers. It will therefore beappreciated that the present invention contemplates cleaving (wherenecessary or desirable) through an adjoining region 160 distant from theactual end of the grating 150 whereby the prior art problems associatedwith cleaving the grating and thereby introducing an uncontrolled phaseshift into the cavity are completely avoided.

A further advantage of the present invention can now be understood. Thepresent invention comprehends a method of manufacturing where there isno need to cleave the individual elements from the wafer, nor is thereany need to complete the end finishing or packaging of the laserstructure before even beginning to test the laser structures forfunctionality. For example, referring to FIG. 1, the electrodes 12, 14are formed into the structure 10 as the structure is built and still ina wafer form. Each of the structures 10 can be electrically isolatedfrom adjacent structures when on wafer, by appropriate patterning anddeposition of electrodes on the wafer, leaving high resistance areas inthe adjoining regions 160 between gratings as noted above. Therefore,electrical properties of each of the structures can be tested on wafer,before any packaging steps occur, simply by injecting current into eachgrating structure 150 on wafer. Thus, defective structures can bediscarded or rejected before any packaging steps are taken (even beforecleaving), meaning that the production of laser structures according tothe present invention is much more efficient and thus less expensivethan in the prior art where packaging is both more complex and requiredbefore any testing can occur. Thus cleaving, packaging and end finishingsteps for non-functioning or merely malfunctioning laser structuresrequired in the prior art edge emitting laser manufacture are eliminatedby the present invention.

FIG. 8 shows a further embodiment of the present invention including adetector region 200 located at one side of the grating region. Thedetector region 200 can be made integrally with the laser structure byreverse biasing the layers of the detector region 200 to act as aphotodetector. This detector is inherently aligned with the surfaceemitting laser 10 and is easily integrated by being fabricated at thesame time as the laser structure, making it very cost efficient toinclude. In this way the signal output can be sensed by the detector 200and the quality of the optical signal, in terms of power stability canbe monitored in real time. This monitoring can be used with an externalfeedback loop to adjust a parameter, for example the injection current,which might be varied to control small fluctuations in the power. Such afeedback system allows the present invention to provide very stable orsteady output signals over time, to tune the output signal as requiredor to compensate for changes in environment such as temperature changesand the like which might otherwise cause the output signal to wander.Variations in an output optical signal can be therefore compensated forby changes in a parameter such as the current injected into the laser.In this way, the present invention contemplates a built-in detector forthe purpose of establishing a stable signal source, over a range ofconditions, having a stable output power.

FIG. 9 is a further embodiment of the present invention which includesenhanced confinement of the optical near-field to the central part ofthe device. While a nominal increase in spatial hole-burning isexpected, the offsetting advantage is that the surface emission is morestrongly confined in the dimension along the laser cavity, thusachieving closer to cylindrical symmetry. To achieve this result in thisembodiment, the central part of the laser structure consists of a second(or higher) order grating with a first order grating 300 added to eachend of the second order grating region 24. Separate electrodes 302 and304 are provided to activate the first order grating region 300. Theeffect of the adjacent first order grating beside the second ordergrating is to enhance the confinement of the output signal.

FIG. 10 is a top view of an array of semiconductor laser structures 10according to the present invention all formed on a single commonsubstrate 400. In this case, each grating 24 can be designed to producea specific output (specific signal) in terms of wavelength and outputpower. The present invention contemplates having each of the adjacentsignal sources which form the array at the same wavelength or specificsignal as well as having each of them at a different wavelength orspecific signal. Thus, the present invention contemplates a single arraystructure which simultaneously delivers a spectrum of individualwavelengths suitable for broadband communications from a plurality ofside by side semiconductor laser structures. Each laser structure orsignal source may be independently modulated and then multiplexed into aDWDM signal. Although three are shown for ease of illustration, becauseof the flexibility in design, the array can include from two up to fortyor more individual wavelength signal sources on a common substrate 400.

It will be appreciated by those skilled in the art that while referencehas been made to preferred embodiments of the present invention variousalterations and variations are possible without departing from thespirit of the broad claims attached. Some of these variations have beendiscussed above and others will be apparent to those skilled in the art.For example, while preferred structures are shown for the layers of thesemiconductor laser structure of the invention other structures may alsobe used which yield acceptable results. Such structures may be eitherloss coupled or gain coupled as shown. What is believed important is tohave a duty cycle in the grating at less than 50% and most preferablyclose to 25%.

1. A surface emitting semiconductor laser comprising: a semiconductorlasing structure having an active layer, opposed cladding layerscontiguous to said active layer, a substrate, a refractive indexstructure to laterally confine an optical mode volume and electrodes bywhich current can be injected into said semiconductor lasing structure,and a second or higher order distributed diffraction grating havingperiodically alternating grating elements, each of said grating elementsbeing characterized as being either a high gain element or a low gainelement, where, upon current injection, the low gain element exhibitslow gain, no gain or absorption as compared to the high gain element,each of said elements having a length, the length of the high gainelement and the length of the low gain element together defining agrating period, said grating period being in the range required toproduce an optical signal in the optical telecommunications signal band,wherein the length of one of the high gain elements is no more than 0.5times the length of the grating period.
 2. A surface emittingsemiconductor laser as claimed in claim 1 wherein the length of saidhigh gain elements is between 15% and 35% of the length of said gratingperiod.
 3. A surface emitting semiconductor laser as claimed in claim 1wherein the length of one of said high gain elements is about 25% of thelength of said grating period.
 4. A surface emitting semiconductor laseras claimed in claim 1 wherein said distributed diffraction grating isoptically active and is formed in a gain medium in the active layer. 5.A surface emitting semiconductor laser as claimed in claim 1 whereinsaid distributed diffraction grating is optically active and is formedin a loss medium in the mode volume.
 6. A surface emitting semiconductorlaser as claimed in claim 1 wherein said distributed diffraction gratingis not optically active and is formed from a current blocking material.7. A surface emitting semiconductor laser as claimed in claim 1 whereinsaid grating comprises an integral number of grating periods.
 8. Asurface emitting semiconductor laser as claimed in claim 1 wherein saidstructure further includes an adjoining region at least partiallysurrounding said grating in plan view.
 9. A surface emittingsemiconductor laser as claimed in claim 8 wherein said adjoining regionfurther includes integrally formed absorbing regions located at eitherend of said distributed diffraction grating.
 10. A surface emittingsemiconductor laser as claimed in claim 1 further including an adjoiningregion having a photodetector.
 11. A surface emitting semiconductorlaser as claimed in claim 10 wherein said photodetector is integrallyformed with said lasing structure.
 12. A surface emitting semiconductorlaser as claimed in claim 11 further including a feedback loop connectedto said photodetector to compare a detected output signal with a desiredoutput signal.
 13. A surface emitting semiconductor laser as claimed inclaim 12 further including an adjuster for adjusting an input current tomaintain said output signal at a desired characteristic.
 14. A surfaceemitting semiconductor laser as claimed in claim 8 wherein saidadjoining region is formed from a material having a resistancesufficient to electrically isolate said grating, when said laser is inuse.
 15. A surface emitting laser as claimed in claim 1 wherein one ofsaid electrodes includes a signal emitting opening.
 16. A surfaceemitting laser as claimed in claim 1 wherein said laterally confiningrefractive index structure is one of a ridge waveguide or a buriedheterostructure waveguide.
 17. A surface emitting semiconductor laser asclaimed in claim 8 wherein said laser structure further includes alongitudinal field confinement structure at either end of said lasercavity.
 18. A surface emitting semiconductor laser as claimed in claim17 wherein said longitudinal field confinement structure comprises anintegrally formed first order grating, and, said laser further includessecond electrodes associated with said first order grating to inject acurrent therein.
 19. An array of surface emitting semiconductor lasersas claimed in claim 1 wherein said array includes two or more of saidlasers on a common substrate.
 20. An array of surface emittingsemiconductor lasers as claimed in claim 19 wherein each of said two ormore of said lasers produces an output signal having a differentwavelength and output power and can be individually modulated.
 21. Anarray of surface emitting semiconductor lasers as claimed in claim 19wherein each of said two or more of said lasers produces an outputsignal having the same wavelength.
 22. A method of fabricating surfaceemitting semiconductor lasers, said method comprising the steps of:forming a plurality of semiconductor laser structures by forming, insuccessive layers on a common wafer substrate; a first cladding layer,an active layer and a second cladding layer on said wafer substrate;forming a plurality of second or higher order distributed diffractiongratings associated with said active layer on said wafer substrate;forming electrodes on each of said semiconductor laser structures onsaid wafer substrate for injecting current into each of said gratings,where one of said electrodes has an aperture to allow light emission;and testing each of said semiconductor laser structures by injecting atesting current into said structures while the same are still connectedto said common wafer substrate.
 23. A method of fabricating surfaceemitting semiconductor lasers as claimed in claim 22 further comprisingthe step of simultaneously forming adjoining regions between saidplurality of distributed diffraction gratings.
 24. A method offabricating surface emitting semiconductor lasers as claimed in claim 22further including the step of providing a refractive index structure tolaterally confine an optical mode of each of said semiconductor laserstructures in the form of a ridge waveguide or a buried heterostructurewaveguide.
 25. A method of fabricating surface emitting semiconductorlasers as claimed in claim 22 further including the step of forming ateither end of each of said gratings an absorbing region in saidadjoining region.
 26. A method of fabricating surface emittingsemiconductor lasers as claimed in claim 22 further including the stepof cleaving said wafer along said adjoining regions to form an array oflasers.
 27. A surface emitting semiconductor laser comprising: asemiconductor lasing structure having an active layer, opposed claddinglayers contiguous to said active layer, a substrate, a refractive indexstructure to laterally confine an optical mode volume and electrodes bywhich current can be injected into said semiconductor lasing structure,and a second or higher order distributed diffraction grating associatedwith an active layer of said lasing structure, said distributeddiffraction grating having periodically alternating grating elements,each of said grating elements having a gain effect wherein any adjacentpair of grating elements includes one element having a relatively highgain effect and one having a relatively low gain effect wherein, adifference in such gain effects, the different refractive indices of thehigh and low gain elements, and the grating period cause an outputsignal in the range near 850 nm, or 910 nm to 990 nm, or 1200 nm to 1700nm and wherein each of said grating elements has a length, the length ofthe relatively high gain effect element and the length of the relativelylow gain effect element together defining a grating period, wherein thelength of one of the relatively high gain elements is no more than 0.5times the length of the grating period.
 28. A surface emittingsemiconductor laser as claimed in claim 27 wherein said laterallyconfining refractive index structure is one of a ridge waveguide or aburied heterostructure waveguide.
 29. A method of stabilizing an outputsignal from a laser comprising the steps of: energizing a surfaceemitting laser by injecting current into the laser; energizing one ormore associated photodetectors associated with the laser; monitoring thequality of the output signal from the surface emitting laser with thephotodetector; and adjusting the amount of current injected into thelaser to prevent signal wandering.
 30. The method of claim 29 furtherincluding a pre-step of forming said photodetector integrally with saidlaser.
 31. A method of stabilizing an output signal from a laser asclaimed in claim 30 further including the step of connecting saidphotodetector to a feedback loop and comparing said detected signaloutput with a desired signal output.
 32. A method of stabilizing anoutput signal from a laser as claimed in claim 31 further including thestep of providing an adjuster and adjusting the amount of currentinjected into said laser to prevent signal wandering in response to saidcomparison of arising from said feedback loop.
 33. A surface emittingsemiconductor laser for producing output signals of defined spatialcharacteristics said laser comprising; a semiconductor lasing structurehaving an active layer, opposed cladding layers contiguous to saidactive layer, a substrate and electrodes by which current can beinjected into said semiconductor lasing structure to produce an outputsignal in a telecommunications band and a second or higher orderdistributed diffraction grating sized and shaped to provide, upon theinjection of current into the lasing structure, a lower gain thresholdto a single lobed mode than the gain threshold provided to any othermode wherein said single lobe mode lases to facilitate coupling saidoutput signal to an optical fibre.
 34. A surface emitting semiconductorlaser for producing output signals of defined spatial characteristics asclaimed in claim 33 wherein said distributed diffraction grating iscomprised of alternating grating elements which define a grating period,wherein one of said elements is a relatively high gain element and theadjacent element is a relatively low gain element and wherein the lengthof the relatively high gain element is no more than 0.5 times the lengthof the grating period.
 35. A surface emitting semiconductor laser forproducing output signals of defined spatial characteristics as claimedin claim 33 wherein said distributed diffraction grating is a gaincoupled grating in an active region of said structure.
 36. A surfaceemitting semiconductor laser for producing output signals of definedspatial characteristics as claimed in claim 33 wherein said distributeddiffraction grating is loss coupled grating in the mode volume of saidstructure.
 37. A surface emitting semiconductor laser for producingoutput signals of defined spatial characteristics as claimed in claim 33wherein said distributed diffraction grating is a current blockinggrating in said semiconductor lasing structure.